1. Field of the Invention
The present invention relates to a circuit for encoding and decoding key input data, such as for a remote control keyboard circuit, from a matrix of switches with a single transmission wire, including circuitry for self calibration.
2. Description of the related art including information disclosed under 37 CFR .sctn..sctn. 1.97-1.99.
Heretofore, various circuits for encoding the position of multiple switches in keyboard circuits for remote controls have been proposed. Several examples of analogous and non-analogous circuits for encoding and/or decoding keyboard data for remote controls are disclosed in the following U.S. Patents:
______________________________________ U.S. Pat. No. Patentee ______________________________________ 4,015,254 Strandt 4,118,700 Leniham 4,580,138 Morrison 4,695,840 Darilek 4,817,010 Dobbins 4,884,070 Hannaford ______________________________________
The Strandt, U.S. Pat. No. 4,015,254 discloses a keyboard encoding circuit utilizing an A/D converter having a group of resistors coupling in series a positive voltage reference and circuit ground. The resistors in series establish a voltage divider network from which multiple voltages are generated. A group of switches are commonly connected to the input of an analog-to-digital converter and uniquely connected to one of the multiple voltages generated by the voltage divider network such that when one of the switches is closed, the voltage associated with the switch is applied to the input of the analog-to-digital converter which decodes which switch was pressed.
The Leniham, U.S. Pat. No. 4,118,700 discloses a circuit with single wire transmission of multiple switch operations. Each of a plurality of switches is connected in parallel with an impedance and the switches are coupled to form a series circuit. A reference voltage is coupled to one end of the series circuit with a termination impedance coupled to the other end to form a voltage divider. Operating a combination of switches will produce a divided voltage which is coupled to an analog-to-digital converter to decode the combination of switches operated.
The Morrison, U.S. Pat. No. 4,580,138 discloses a circuit for monitoring the state of each of a group of switches. Each switch is connected in parallel with a resistor. All of the switches are connected in series, with each end of the series string of switches being connected to an analog-to-digital converter for decoding the combination of switches depressed. The resistor values are computed by multiplying powers of 2 with a base resistor value. By computing the resistor values this way, each combination of switch closures will result in a unique resistance seen by the analog-to-digital converter.
The Darilek, U.S. Pat. No. 4,695,840 discloses a circuit for determining the position of a group of remote switches using duty cycle modulation. Each switch is connected in series with a resistor. Each series combination switch and resistor is connected in parallel with all of the other series switch and resistor combinations. The resistor values are computed by multiplying powers of 2 with a base resistor value. The group of remote switches are used to set the current for a constant current source. A timing mechanism, responsive to the constant current source, triggers the switching mechanism to generate a signal having a duty cycle that is dependant on the switches depressed.
The Dobbins, U.S. Pat. No. 4,817,010 discloses a circuit for switch detection and decoding for a vending machine. The circuit contains a group of resistor in series. Each end of the series group of resistors is connected to the different inputs of a 555 timer. One of the ends of the series group of resistors also is connected to a supply voltage. Connected between each adjacent pair of resistors is a switch with the other end of the switch being connected to the supply voltage. Dependant on which switch is closed a different aggregate resistance will be connected to the inputs of the 555 timer. The 555 timer generates a signal with a frequency dependant on the resistance at the inputs, which is then decoded by the microprocessor to determine which switch has been actuated.
The Hannaford, U.S. Pat. No. 4,884,070 discloses a circuit for reading the state of a plurality of switches. Each switch is connected in parallel with a resistor. All of the switches are connected in series. The ends of the series connection of switches are connected to both a constant current driver and an analog-to-digital converter. The resistors are weighted as a function of a power of the same base. When a switch is actuated the resistor in parallel with the actuated switch is shunted across, such that the value of the shunted resistor is not relevant to the aggregate resistance of the resistors/switches in series. The constant current driver produces a voltage drop across the resistors/switches in series dependant on its aggregate resistance. The voltage drop is decoded by the analog-to-digital converter to determine which combination of switches have been depressed.